Development and Application of the Community-enabled Lifecycle Analysis of Stormwater Infrastructure Costs (CLASIC) Tool

Abstract

Urbanization alters natural hydrologic processes by increasing impervious surface area, reducing infiltration and evapotranspiration, increasing runoff volume and peak flow, while transporting pollutants to receiving waters. Municipalities are increasingly expected to manage these impacts while also addressing aging infrastructure, changing land use, regulatory requirements, limited budgets, and future climate uncertainty. Stormwater control measures (SCMs), including green, gray, and hybrid green-gray infrastructure, provide a means of reducing these impacts by capturing, infiltrating, detaining, treating, or conveying stormwater runoff. However, planning for SCM implementation at the municipal scale remains difficult as a result of the level of complexity needed for detailed engineering models. These models often require extensive site-specific information, drainage infrastructure data, calibration, and technical expertise. At the same time, simplified site-scale tools often do not provide the spatial scale, scenario flexibility, life-cycle cost analysis, water quality analysis, and co-benefit evaluation needed to support municipal stormwater planning. This dissertation addresses these limitations through the development and application of the Community-enabled Lifecycle Analysis of Stormwater Infrastructure Costs (CLASIC) tool, a web-based decision-support framework for evaluating stormwater infrastructure scenarios at neighborhood to municipal scales. The first objective of this research was to develop and evaluate a simplified hydrologic modeling approach capable of supporting municipal-scale analysis with reduced input data requirements and computational burden. A modified version of the U.S. Environmental Protection Agency (USEPA) Storm Water Management Model (SWMM), referred to as SWMM for Low Impact Technology Evaluation (SWMM-LITE), was developed to estimate hydrologic outputs at the planning-level without requiring a complete hydraulic drainage network. SWMM-LITE simplifies the representation of subcatchments and removes hydraulic routing elements that are necessary for detailed flood analysis but not necessarily required for estimating annual runoff, infiltration, and evaporation. The approach was evaluated using the McClelland basin in Fort Collins, Colorado, where outputs from SWMM-LITE were compared with a detailed SWMM model and the National Stormwater Calculator. Three scenarios were analyzed, including a baseline condition without SCMs and two scenarios that included differing SCM implementation. Across the three scenarios, SWMM-LITE estimates of annual average runoff, infiltration, and evaporation were within ±0.1% of the detailed SWMM model for a 30-year continuous simulation. For 2-year, 10-year, and 100-year design storm events, hydrologic outputs from SWMM-LITE differed from SWMM by less than ±2.5%. A sensitivity analysis further showed that the most influential parameters were generally consistent between SWMM and SWMM-LITE. These results demonstrate that SWMM-LITE can provide reliable planning-level estimates of hydrologic performance for evaluating SCM scenarios at municipal scales. The second objective was to develop a methodology for representing SCMs within a municipal-scale decision-support framework without requiring detailed site design information for each individual facility. SCMs vary widely in physical configuration, hydrologic function, treatment mechanism, maintenance requirements, cost, and potential co-benefits. Final design typically requires information such as tributary area, available footprint, storage depth, individual component design, soil infiltration capacity, overflow location, groundwater separation, utility conflicts, maintenance access, and discharge constraints. Much of this information is unavailable during early planning and is infeasible to collect when evaluating several SCMs. To address this challenge, a technology-unit framework was developed to represent SCMs using planning-level information for a planning-level analysis. Each technology unit represents either a discrete SCM facility, such as a rain garden, detention basin, wet pond, stormwater harvesting cistern, storage vault, sand filter, or infiltration trench, or a unit area of an area-based practice, such as permeable pavement, green roof, or disconnection. The framework allows users to define SCM scenarios based on technology type, treated impervious area, design capture depth, technology size, and selected design characteristics that influence hydrologic performance, water quality, life-cycle cost, and co-benefit scoring. The technology-unit framework was integrated with SWMM-LITE by partitioning study areas into directly connected impervious area, separate pervious area, captured impervious area, and receiving treatment areas. Volume-based technologies are sized based on the impervious area treated and the design capture depth, while area-based technologies are sized based on a defined relationship between technology area and treated impervious area. Water quality effects are estimated using runoff volumes, land-use-based wash-off concentrations, and technology-specific effluent concentrations. Life-cycle costs are estimated using line-item construction, maintenance, and rehabilitation cost build ups that vary by technology type, size, region, and user-selected design parameters. Co-benefits are evaluated using economic, social, and environmental indicators to provide comparative planning-level estimates of potential triple-bottom-line benefits. A demonstration using stormwater infrastructure data from Longmont, Colorado, showed that representative technology units can reasonably approximate municipal SCM implementation using limited planning-level information. Representative rain garden and detention basin units closely matched estimated treated impervious area and capture volume from the municipal inventory, although individual average technology surface areas varied between the representative technology and the summary of actual technologies. This confirmed the intended use of the methodology as a planning-level representation of SCM deployment rather than a replacement for final engineering design. The third objective was to integrate the hydrologic model and SCM representation methodology into the CLASIC web-based decision-support tool. CLASIC was developed to support municipal stormwater planning by allowing users to define a project area, access national datasets, modify model assumptions, build alternative scenarios, evaluate water quality and hydrologic performance, estimate life-cycle costs, assess co-benefits, set performance and cost targets, and compare outputs across scenarios. The tool incorporates national datasets for land cover, soils, slope, climate, and water quality, while allowing users to modify default assumptions where local data are available. CLASIC supports ten widely used green, gray, and hybrid stormwater technologies and evaluates scenarios using multiple outputs, including runoff volume, infiltrated volume, evaporated volume, pollutant loads and reductions, construction cost, maintenance cost, rehabilitation cost, present value life-cycle cost, and social, economic, and environmental co-benefit scores. The resulting framework provides a single platform for comparing stormwater infrastructure alternatives across multiple decision criteria. The final objective was to demonstrate how CLASIC can support municipal-scale stormwater planning through application to Longmont, Colorado. The Longmont application was designed to evaluate how urbanization, existing SCMs, additional SCM implementation, land-use change, and future climate conditions may influence hydrologic performance, water quality, infrastructure costs, and co-benefits. The case study framework includes comparison of pre-development, current-development, existing-SCM, additional-SCM, future-land-use, and future-climate scenarios. This application demonstrates how CLASIC can be used to translate complex stormwater planning questions into scenario-based comparisons that are understandable and useful for municipal decision-makers. Overall, this dissertation advances municipal-scale stormwater planning by developing a simplified hydrologic modeling framework, a scalable SCM representation methodology, and an integrated web-based decision-support tool. SWMM-LITE reduces the data and computational requirements associated with detailed SWMM modeling while preserving the ability to estimate planning-relevant hydrologic outputs. The technology-unit methodology enables SCMs to be represented consistently across large study areas without requiring detailed site design. CLASIC brings these components together with water quality analysis, life-cycle cost estimation, and co-benefit assessment to support comparison of green, gray, and hybrid stormwater infrastructure scenarios. The resulting framework provides municipalities with a practical tool for evaluating stormwater infrastructure investments, understanding tradeoffs among competing objectives, and planning SCM implementation strategies that improve runoff management, pollutant reduction, budget forecasting, and community benefits.